Wireless devices have been in use for many years for enabling mobile communication of voice and data. Such devices can include mobile phones and wireless enabled personal digital assistants (PDA's) for example. FIG. 1 is a generic block diagram of the core components of such wireless devices. The wireless core 10 includes a base band processor 12 for controlling application specific functions of the wireless device and for providing and receiving voice or data signals to a radio frequency (RF) transceiver chip 14. The RF transceiver chip 14 is responsible for frequency up-conversion of transmission signals, and frequency down-conversion of received signals. RF transceiver chip 14 includes a receiver core 16 connected to an antenna 18 for receiving transmitted signals from a base station or another mobile device, and a transmitter core 20 for transmitting signals through the antenna 18 via a gain circuit 22. Those of skill in the art should understand that FIG. 1 is a simplified block diagram, and can include other functional blocks that may be necessary to enable proper operation or functionality.
Generally, the transmitter core 20 is responsible for up-converting electromagnetic signals from base band to higher frequencies for transmission, while receiver core 16 is responsible for down-converting those high frequencies back to their original frequency band when they reach the receiver, processes known as up-conversion and down-conversion (or modulation and demodulation) respectively. The original (or base band) signal, may be, for example, data, voice or video. These base band signals may be produced by transducers such as microphones or video cameras, be computer generated, or transferred from an electronic storage device. In general, the high frequencies provide longer range and higher capacity channels than base band signals, and because high frequency radio frequency (RF) signals can propagate through the air, they are preferably used for wireless transmissions.
All of these signals are generally referred to as radio frequency (RF) signals, which are electromagnetic signals; that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation.
In certain communications standards such as the well known GSM standard, various modulation techniques are available for transmitting data. One such technique is GMSK (Gaussian Minimum Shift Keying) modulation, which is generally preferred over other modulation techniques due to its relatively higher spectral efficiency and reduced side-band power. Generally, GMSK modulation is a form of frequency modulation for coding bits of data being transmitted, and other frequency modulation techniques can be used instead of GMSK. For any frequency modulation technique, the transmitter core 20 includes a frequency synthesizer circuit for generating the modulated frequency signal to be used in downstream transmit circuits of the transmitter core 20.
FIG. 2 is a block diagram of a prior art frequency synthesizer that is used for generating a modulated frequency output signal Fout in response to a reference clock signal CLK_ref and a modulation signal MOD_WD. Frequency synthesizer 30 includes a loop consisting of a phase frequency detector (PFD) 32, a charge pump 34, an analog filter 36, a voltage controlled oscillator (VCO) 38, and a frequency divider 40. The PFD 32 determines the difference in phase between CLK_ref and the feedback signal from frequency divider 40, which is used by charge pump 34 to generate a charge corresponding to this difference. The analog filter restricts bandwidth of the closed loop system and filters out unwanted noise. The VCO 38 phase is adjusted by the charge provided by charge pump 34. The frequency divider 40 then divides Fout by a value N, which is generally set by the base band processor for the application. Assuming N does not change, the loop will eventually lock such that the phase of Fout is matched to the phase of CLK_ref. For frequency modulation applications, a well known sigma-delta modulator 42 is provided for receiving a digital word that adjusts the value of N of frequency divider 40. This digital word is provided at a frequency necessary for modulating Fout.
One of the issues of the frequency synthesizer 30 is that the circuit has a particular closed loop frequency response, primarily due to the presence of analog components such as charge pump 34, analog filter 36 and VCO 38 which are each sensitive to process, voltage and temperature (PVT) variations. FIG. 3A is a graph of an example closed loop frequency response for frequency synthesizer 30. If this closed loop frequency response is known through simulation or calculations based on the circuit design, then the frequency at which new modulation words can be applied to sigma-delta modulator 42 should be less than fL. Frequency fL is the loop bandwidth of the frequency synthesizer. However, any tone lying beyond fL will have its amplitude attenuated by the closed loop frequency response of the circuit. Those skilled in the art should understand that the gain should remain substantially constant during frequency modulation. Unfortunately, fL is preferably minimized to minimize noise in the circuit, which thereby restricts the rate of modulation that is allowed, which impacts performance of the circuit and the wireless system.
The known solution to this problem is to add a pre-emphasis filter 44 that compensates for the native closed loop frequency response of frequency synthesizer 30 beyond fL. FIG. 3B is a graph of an example filter response for pre-emphasis filter 44. In simplified terms, pre-emphasis filter 44 amplifies the received signal by a predetermined factor before it is received by sigma-delta modulator 42. Therefore, the signal is “boosted” and then attenuated by the closed loop frequency response of frequency synthesizer 30. The net change should be substantially zero. Therefore, the rate of modulation can be increased beyond fL while maintaining a constant amplitude of Fout.
The problem with this system is determining the parameters of pre-emphasis filter 44 that complements the actual closed loop frequency response of frequency synthesizer 30. A theoretically calculated closed loop frequency response of frequency synthesizer 30 can be made prior to fabrication, but the aforementioned PVT sensitivity of the analog circuits will change the actual closed loop frequency response. Fabricated batches of devices can vary from each other due to process variation, and even devices within the same fabricated batch of devices can vary from each other due to process variation. Compounding this problem is the fact that voltage and temperature can change at any time while the wireless device is turned on. Therefore, a theoretically calculated filter response is of little use for designing the proper pre-emphasis filter 44 before fabrication.
It is, therefore, desirable to provide a frequency synthesizer circuit that compensates for closed loop frequency responses due to PVT variation.